Am I right in assuming that you are using shopify?
You can do this by clicking on Shopify - in the navigation to your right.
In the newly opened window you will find your shops listed. Click on edit for the shop where you want to change this.
This will open your shop integration where you will find the option integrated payform which should be on if you are using our payment page.
There should be a little option called customize where you should find information on how to edit the CSS to achieve the desired effect.
Let me know if you need help with this.